1. Field of the Invention
The present invention relates to an instrument for characterizing an optical system. It also relates to a characterization method implemented in this instrument.
2. Prior Art
There are already several instruments for characterizing an optical system. It is in fact possible to identify two families of instruments: systems based on measurement of the wave front (measurement of the aberrations) and other instruments that are not based on wave front measurement and do not allow the measurement of the optical aberrations of the system to be characterized.
Phase measurement systems include:                several phase measurement technologies: Fizeau interferometer, Hartmann or Shack Hartmann technology, lateral-shift interferometer technology;        phase measurement systems making it possible to characterize the aberrations of a system. On the basis of this measurement it is generally possible to determine, in addition to the aberrations, the point spread function (PSF) and the modulation transfer function (MTF) of the system to be characterized. With certain particular implementations, it is also possible to go back to the back focal distance and the numerical aperture of the system to be characterized.        
On the other hand, these systems do not generally give the measurement of the focal length of the system to be characterized or the chromatic aberration value, and are all limited, owing to their implementation, by numerical aperture. Moreover, these systems measure only one field at a time and do not allow the measurement of the field curvature or the distortion of the objective.
Systems that are not based on phase measurement include:                certain instruments dedicated to the measurement of the focal length of the objective. But these are not based on measurement of the wave front;        other instruments dedicated to the measurement of the MTF of the objectives. In this family, certain instruments allow measurement of the objective at several angles of view, with the feature of taking account, in the measurement, of the effect of the field curvature of the objective. However, the capacity of these instruments is limited by their inability to measure the optical aberrations of the system to be characterized (these instruments are not based on wave front measurement);        other systems allowing measurement of the vignetting and the relative intensity as a function of the angle of view of an objective to be characterized but which do not provide the other data on the optical quality;        and        instruments allowing the measurement of the distortion of an objective to be characterized but not providing the other data on the optical quality.        
The purpose of the present invention is to overcome the drawbacks and limitations of the abovementioned characterization instruments, by proposing an optical metrology system based on a wave front measurement allowing the characterization of optical components having a positive (convergent) power without being limited by numerical aperture.
It thus relates to a novel concept for a characterization and metrology instrument which is not limited by numerical aperture and obtains an adequate set of data on the optical quality of the characterized system.